The present invention generally relates to the field of magnetic memories. More particularly, the present invention relates to a magnetic memory device and method wherein a magnetic field is modified to compensate for temperature variations in a magnetic memory cell.
Magnetic random access memory (MRAM) is a type of non-volatile magnetic memory which includes magnetic memory cells. A typical magnetic memory cell includes a layer of magnetic film in which the magnetization of the magnetic film is alterable and a layer of magnetic film in which magnetization is fixed or xe2x80x9cpinnedxe2x80x9d in a particular direction. The magnetic film having alterable magnetization is typically referred to as a data storage layer, and the magnetic film which is pinned is typically referred to as a reference layer.
A magnetic memory cell is usually written to a desired logic state by applying external magnetic fields that rotate the orientation of magnetization in its data storage layer. The logic state of a magnetic memory cell is indicated by its resistance which depends on the relative orientations of magnetization in its data storage and reference layers. The magnetization orientation of the magnetic memory cell assumes one of two stable orientations at any given time. These two stable orientations, parallel and anti-parallel, represent, for example, logic values of xe2x80x9c0xe2x80x9d and xe2x80x9c1.xe2x80x9d
Typically, the orientation of magnetization in the data storage layer aligns along an axis of the data storage layer that is commonly referred to as its easy axis. The external magnetic fields are applied to flip the orientation of magnetization in the data storage layer along its easy axis to either a parallel or anti-parallel orientation. With parallel orientation, the magnetic memory cell is in a low resistance state because the orientation of magnetization in its data storage layer is substantially parallel along the easy axis. With anti-parallel orientation, the magnetic memory cell is in a high resistance state because the orientation of magnetization in its data storage layer is substantially anti-parallel along the easy axis.
A typical magnetic memory includes an array of magnetic memory cells. Word lines extend along rows of the magnetic memory cells, and bit lines extend along columns of the magnetic memory cells. Each magnetic memory cell is located at an intersection of a word line and a bit line. A selected magnetic memory cell is usually written by applying electrical currents to the particular word and bit lines that intersect at the selected magnetic memory cell. The electrical current applied to the particular bit line generates a magnetic field substantially aligned along the easy axis of the selected magnetic memory cell. This magnetic field may be referred to as a bit line write field. An electrical current applied to the particular word line also generates a magnetic field substantially perpendicular to the easy axis of the selected magnetic memory cell. This magnetic field may be referred to as a word line write field. The sum of the bit line write field and the word line write field must be greater than a critical switching field or write threshold to enable the magnetization in the data storage layer to change and align according to the applied write fields. A magnetic memory cell receiving only the word line or the bit line write field is termed a half-selected magnetic memory cell. The magnitudes of the word line and bit line write fields are usually chosen to be high enough so that the magnetization in the data storage layer of the selected magnetic memory cell changes and aligns according to the applied write fields, but not too high so that the half-selected magnetic memory cells which are subject to either the word line or the bit line write field do not change their direction of magnetization in the data storage layer.
One problem that can occur in the magnetic memory is temperature variations within the array of magnetic memory cells. Operation of the array or changes in the ambient temperature can cause the temperature of the magnetic memory cells to vary, which in turn causes the coercivity of the magnetic memory cells to change. The coercivity of the magnetic memory cells decreases with increasing temperature resulting in a decrease in the critical switching field. Increasing temperatures can increase the likelihood that either the bit line write field or the word line write field will be high enough to cause half-select switching of magnetic memory cells. Conversely, decreasing temperatures increase the likelihood that the sum of the bit line write field and the word line write field will not be higher than the critical switching field required to switch the magnetic memory cells.
The present invention provides a magnetic memory. In one embodiment, the magnetic memory includes a magnetic memory cell, a conductor which crosses the magnetic memory cell and a circuit coupled to the conductor configured to apply a modified magnetic field to the magnetic memory cell in response to temperature variations in the magnetic memory cell.